1. Technical Field
The invention relates to a display device comprising a display panel having a first light-transmissive substrate provided with electrodes at the area of pixels arranged in rows and columns, a second light-transmissive substrate and liquid crystalline material between the two substrates, an illumination system situated on the side of the second substrate remote from the liquid crystalline material, said illumination system comprising a backlight and an optical shutter element provided with means for selectively transmitting light for a group of rows of pixels or a group of columns of pixels.
Display devices of this type are used in, for example, portable apparatus such as laptop computers, mobile telephones, personal organizers etc., but also in, for example, television applications.
2. Description
A display device of the type mentioned above is described in U.S. Pat. No. 5,103,328. This document shows a liquid crystal switch built up of separately switchable segments (optical shutter elements) between a flat light source (backlight) and a display panel. The liquid crystal switch is adapted in such a way that a plurality of rows of pixels of the display panel corresponds to one segment and its function is to shield pixels that are written in from the light coming from the backlight.
Each segment is coupled to one part of the switch, which part switches separately. By consecutively illuminating different, uninterrupted rows of pixels of the display panel via the associated part of the switch in such a device, scanning window applications are possible, in which the light from the backlight is presented in the form of strips. The light source emits unpolarized light which, when using, for example, a liquid crystal switch based on the twisted-nematic liquid crystal effect, is incident on a polarizer upon entrance into the switch. Polarized light of the desired polarization is passed; the other light is absorbed in the polarizer. Here, a loss of light of about 50% occurs. The light of the correct polarization is subsequently only passed by the switch segment which is opened so that a large part of this light (about 80% when using 5 segments) is absorbed in the switch (notably in the analyzer).
It is, inter alia, an object of the present invention to obviate the above-mentioned drawbacks completely or partly. To this end, a display device according to the invention is characterized in that the display device comprises at least one reflective polarizer in the light path between the backlight and the display panel.
In this application, a xe2x80x9creflective polarizerxe2x80x9d is understood to mean a polarizer which, on at least one side of the reflective polarizer, transmits linearly polarized radiation (light) in the direction of polarization of the reflective polarizer and reflects linearly polarized radiation (light) perpendicularly to this direction.
Polarized light of the desired direction of polarization is transmitted by the optical shutter element at the area of the switch segment which is opened. The reflective polarizer is situated, for example, parallel to an exit face of the backlight or of an optical waveguide of optically transparent material associated with the backlight, with an exit face facing the display panel. The transmitted light preferably has the same direction of polarization as that of a polarizer on the entrance face of the display panel. This light is subsequently modulated in the display panel in which, for example, the polarization changes, which is made visible by means of a second polarizer or analyzer.
At other areas, notably when the optical shutter element is based on a birefringent effect, polarized light having a different direction of polarization is transmitted, which light is absorbed by the polarizer on the entrance face of the display panel. The light loss thereby occurring is prevented in a preferred embodiment according to the invention, which is characterized in that the display device comprises, parallel to the exit face, a second reflective polarizer between the optical shutter element and the display panel.
In a further variant, the optical shutter element is situated between the display panel and the second reflective polarizer.
Said switches usually cover a surface area having the size of the surface area of the display panel. Also the light from the backlight must cover this surface area, which does not only require more material but also imposes stringent requirements on the quality of said backlight, due to the desired uniformity. To avoid this, an advantageous embodiment of a display device according to the invention is characterized in that the optical waveguide is provided with means for coupling in light in a direction parallel to the exit face.
The light is now coupled into the optical waveguide from, for example, an edge of the optical waveguide. The light beam to be coupled in has considerably smaller dimensions than the surface area of the display panel. This surface area is now defined, for example, by the width of a light strip (which is a part of, for example, the overall height of the display panel) and the thickness of the optical waveguide (which is usually considerably smaller than, for example, the overall width of the display panel). This makes it easier to couple light of one intensity into the optical waveguide (uniform light source).
A first variant is characterized in that the illumination system comprises at least one backlight and an optical waveguide having at least one entrance face for light, while light from the backlight can be coupled in along the entrance face extending substantially transversely to the exit face, and a selectively switchable light switch is situated between the backlight and the entrance face.
The entrance face is situated, for example, along an end face of the optical waveguide extending substantially transversely to the rows, while light from the backlight can be coupled in along this end face. The selectively switchable light switch then comprises, for example, a liquid crystal switching device with a liquid crystal between two substrates, one or both of which are provided with strip-shaped electrodes. The backlight does not only have a much smaller surface area than in the conventional display device, but the light switch is also much smaller and can therefore be manufactured at lower cost. Consequently, the backlight and the light switch can be easily integrated to one assembly. Substitution of such a combination is simpler than in the known device because alignment in only one dimension is necessary. The smaller dimension also reduces the duration of the filling process. Also the use of thinner optically switchable layers is possible, so that the switching time is faster.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.